1. Field of the Invention
The invention relates to a gas chromatograph for analyzing a gas mixture comprising at least one separation column for separating components of a sample of the gas mixture which is fed through the separation column by a carrier gas, a thermal conductivity detector that has a sensing element arranged downstream from the separation column and has a first operating temperature, where the thermal conductivity detector is further configured to detect the separated components in a non-destructive manner and to generate a detector signal in response to each of the detected components, at least one further thermal conductivity detector that has a further sensing element arranged downstream or upstream from the thermal conductivity detector and which is configured to detect separated components and to generate a further detector signal in response to each of the detected components, and an evaluation unit for evaluating the detector signals and further detector signals to determine the concentrations of the detected components.
2. Description of the Related Art
Gas chromatographs are known from WO 03/083467 A2 or US 2005/0123452 A1. Conventional gas chromatographs have several separation columns coupled directly or by a valveless controllable changeover arranged in series. Each separation column is followed by an inline thermal conductivity detector for detecting gas components sufficiently separated up to that point. The thermal conductivity detectors have micro-machined sensing elements comprising micro-machined devices with heated filaments along the axis of a tubular channel. The inner diameters of the channels correspond at least approximately to those of the separation columns so that the sample of the gas mixture is not disturbed at the detector sites. Each sensing element preferably has two inline filaments. These two filaments are diagonally arranged in a Wheatstone bridge together with two filaments of the sensing element of another thermal conductivity detector through which, at the time of the detection, the carrier gas flows.
JP 9 178721 A discloses a gas chromatograph in which a thermal conductivity detector is immediately followed by a flame ionization detector. The thermal conductivity detector is adapted to determine hydrogen and C1 and C2 hydrocarbons, whereas the flame ionization detector is adapted to determine hydrogen and C3 hydrocarbons.
US 2012/0024043 A1 discloses a gas chromatograph with a thermal conductivity detector, another non-destructive detector and a destructive detector coupled in series. The thermal conductivity detector includes sensors for determining properties of the analyte such as a rate of flow, temperature, and/or pressure. The following detectors allow for additional measurement and/or analysis after the thermal conductivity detector determines one or more properties associated with the analyte.
U.S. Pat. No. 4,741,198 A discloses a gas chromatograph where two sensors of a thermal conductivity sensor assembly are disposed in separate cells with one sensor operating at a lower temperature than the other sensor. High concentration samples can be passed through the cell in which the sensor is at the lower temperature while samples with low concentrations of the test gas are passed through the sensor operating at the higher temperature.
Process gas chromatographs (PGCs), as above-mentioned, are often used to monitor a chemical or petrochemical process to ensure the stability of the process and/or the quality of the products from the process. Thermal conductivity detectors are commonly used in PGCs to measure concentrations of gas components eluting from the separation columns. Depending on the nature of a PGC application, many components of widely different concentration ranges can be present in a same analysis cycle and widely different concentration ranges of a same component can occur at different analysis cycles. A higher temperature of the sensing elements of a TCD is often used to improve detector sensitivity for the components of lower concentrations, and a lower temperature is often used to improve detector linear ranges for the components of higher concentrations. However, as only one temperature can be used in a conventional TCD to cover different concentration ranges, a compromised mid-range temperature must often be found, resulting in decreases in both detector sensitivity and linear range and thus sacrificing result accuracy for some components.